Corrosion prevention method for aluminum-based metal component, and corrosion-proof aluminum-based metal product produced by the method

ABSTRACT

The present invention provides a corrosion prevention method for an aluminum-based metal component, which makes it easy to check the presence or absence of, a chemically formed film on the component, and a corrosion-proof aluminum-based metal product produced by the method. In the method, the following treatment agent (A) is brought into contact with a surface of the aluminum-based metal component to chemically form the following film (B) on the surface of the aluminum-based metal component:
     (A) a treatment agent comprising an aqueous solution of zinc sulfate, an aqueous solution of ammonium fluoride and an aqueous solution of sodium molybdate which are mixed together;   (B) a chemically formed film (B) comprising zinc fluoride, molybdenum and aluminum fluoride. Thus, the corrosion-proof aluminum-based metal product is produced.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a corrosion prevention method for analuminum-based metal component made of aluminum or an aluminum alloy,and a corrosion-proof aluminum-based metal product produced by themethod.

2. Description of the Related Art

In the automotive field, for example, an attempt is currently made atweight reduction of motor vehicles for reduction of fuel costs (or forimprovement in ecological performance). For the weight reduction,aluminum or an aluminum alloy is employed instead of iron as a metalmaterial for automotive components such as suspension arms, bushings,engines, engine mounts and body plates.

Under such circumstances, it is initially a common practice tochemically form films on aluminum and aluminum alloy components by achemical treatment (surface treatment) such as a chromate treatment forprevention of corrosion of the components. A treatment agent for thechromate treatment contains hexavalent chromium, which adversely affectshuman bodies and the environment, thereby presenting problems.

Therefore, a recent trend is to refrain from the use of the hexavalentchromium. The applicant of the present invention previously proposed acorrosion prevention method as an alternative to the chromate treatment(see Japanese Unexamined Patent Publication No. 2001-47515) In thismethod, an aluminum-based metal component made of aluminum or analuminum alloy is etched with an acid, an alkali or a fluoride, and thenchemically formed with a film by a chemical treatment using an alkalimetal phosphate.

However, the film chemically formed by the prior art method is pale incolor or colorless and transparent, making it difficult to check whetheror not the component has been subjected to the chemical treatment (filmforming process). This requires improvement from the viewpoint ofproduction control.

In view of the foregoing, it is an object of the present invention toprovide a corrosion prevention method for an aluminum-based metalcomponent, which makes it easy to check the presence or absence of achemically formed film on the component, and to provide acorrosion-proof aluminum-based metal product produced by this method.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention to achieve theaforementioned object, there is provided a method of preventingcorrosion of an aluminum-based metal component made of aluminum or analuminum alloy, the method comprising; bringing a treatment agent (A)into contact with a surface of the aluminum-based metal component tochemically form a film (B) on the surface of the aluminum-based metalcomponent; the treatment agent (A) comprising an aqueous solution ofzinc sulfate, an aqueous solution of ammonium fluoride and an aqueoussolution of sodium molybdate which are mixed together; the chemicallyformed film (B) comprising zinc fluoride, molybdenum and aluminumfluoride.

According to a second aspect of the present invention, there is provideda corrosion-proof aluminum-based metal product comprising: analuminum-based metal component made of aluminum or an aluminum alloy;and a film (B) chemically formed on a surface of the component by theaforementioned corrosion prevention method.

The inventors of the present invention conducted a study on a novelchemical film formation method to make it easy to check the presence orabsence of a film chemically formed on a surface of an aluminum-basedmetal component. As a result, the inventors found that the aforesaidchemically formed film (B) is formed on the surface of thealuminum-based metal component by bringing the aforesaid treatment agent(A) into contact with the surface of the component to impart thecomponent with a sufficient anti-corrosive property and that thepresence or absence of the chemically formed film (B) can be easilychecked because the film (B) is colored. Thus, the present invention isattained.

According to the inventive corrosion prevention method for thealuminum-based metal component, the film (B) chemically formed bybringing the treatment agent (A) into contact with the surface of thealuminum-based metal component is colored. This makes it easy to checkthe presence or absence of the film (B) chemically formed on thesurface.

Particularly, where the treatment agent (A) comprises the following zincsulfate aqueous solution (a), the following ammonium fluoride aqueoussolution (b) and the following sodium molybdate aqueous solution (c)which are mixed in a mass ratio of (a)/(b)/(c)=7/3/2, the respectiveaqueous solutions efficiently contribute to a reaction for the formationof the film:

-   (a) an aqueous solution containing 15% by weight of zinc sulfate    heptahydrate;-   (b) an aqueous solution containing 10% by weight of ammonium    fluoride; and-   (c) an aqueous solution containing 5% by weight of sodium molybdate    dihydrate Further, zinc of zinc fluoride, molybdenum and aluminum of    aluminum fluoride are present in the chemically formed film (B) in a    content ratio of zinc/molybdenum/aluminum=5/4/1 on a mass basis.    Thus, the chemically formed film (B) is optimized from the viewpoint    of adhesion and anti-corrosive property.

Where the treatment agent (A) having a mass ratio of (a)/(b)/(c)=7/3/2comprises 35 to 175 g of the solution (a), 15 to 45 g of the solution(b) and 10 to 50 g of the solution (c), the formability of the film (B)is improved. Further, the film (B) is excellent in anti-corrosiveproperty and adhesion.

Where the treatment agent (A) has a temperature of 45 to 65° C. and thesurface of the aluminum-based metal component is kept in contact withthe treatment agent (A) for a period of 1 to 10 minutes, the formabilityof the film (B) is further improved.

The inventive corrosion-proof aluminum-based metal product is producedby the aforementioned corrosion prevention method, and the film (B)chemically formed on the surface of the aluminum-based metal componentis colored. This makes it easy to check the presence or absence of thechemically formed film (B) on the surface.

Particularly, where zinc of zinc fluoride, molybdenum and aluminum ofaluminum fluoride are present in the chemically formed film (S) in acontent ratio of zinc/molybdenum/aluminum=5/4/1 on a mass basis, thechemically formed film (B) is excellent in adhesion, and imparts thealuminum-based metal component with excellent anti-corrosive property.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Embodiments of the present invention will hereinafter be described indetail.

In a corrosion prevention method according to the present invention, atreatment agent to be described later in detail is brought into contactwith a surface of an aluminum-based metal component made of aluminum oran aluminum alloy, whereby a film to be described later in detail ischemically formed on the surface of the component to prevent corrosionof the aluminum-based metal component. The chemically formed film iscolored yellow or sunny yellow. The color makes it easy to check thepresence or absence of the chemically formed film on the surface of thecomponent.

More specifically, the treatment agent is a solution mixture obtained bymixing an aqueous solution of zinc sulfate, an aqueous solution ofammonium fluoride and an aqueous solution of sodium molybdate. Thetreatment agent (solution mixture) is prepared in the following manner.For the formation of the film, an aqueous solution (a) containing 15% byweight of zinc sulfate heptahydrate, an aqueous solution (b) containing10% by weight of ammonium fluoride, and an aqueous solution (c)containing 5% by weight of sodium molybdate dihydrate are used as thezinc sulfate aqueous solution, the ammonium fluoride aqueous solutionand the sodium molybdate aqueous solution, respectively. The aqueoussolutions (a), (b) and (c) respectively having the aforesaidconcentrations are mixed in a mass ratio of (a)/(b)/(c)=7/3/2 for thepreparation of the treatment agent.

The film chemically formed by using the treatment agent thus preparedcontains zinc fluoride, molybdenum and aluminum fluoride (resulting fromreaction between ammonium fluoride and aluminum in the aluminum-basedmetal component). More specifically, zinc of zinc fluoride, molybdenumand aluminum of aluminum fluoride are present in the chemically formedfilm in a content ratio of zinc/molybdenum/aluminum=5/4/1 or in acontent ratio close to this ratio (4-6/3-6/0.5-2) on a mass basis. Thecomposition (content ratio) of the chemically formed film is determinedby ICP (inductively coupled plasma) emission spectrometry.

More specifically, the treatment agent is prepared by mixing 35 to 175 gof the zinc sulfate aqueous solution (a) containing 15% by weight ofzinc sulfate heptahydrate (or 5.25 to 26.25 g of zinc sulfateheptahydrate on a solute basis), 15 to 45 g of the ammonium fluorideaqueous solution (b) containing 10% by weight of ammonium fluoride (or1.5 to 4.5 g of ammonium fluoride on a solute basis) and 10 to 50 g ofthe sodium molybdate aqueous solution (c) containing 5% by weight ofsodium molybdate dihydrate (or 0.5 to 2.5 g of sodium molybdatedihydrate on a solute basis). Thus, the resulting treatment agentcontains the aqueous solution (a), the aqueous solution (b), and theaqueous solution (c) in a mass ratio of (a)/(b)/(c)=7/3/2. In order toimprove the formability of the film, the temperature of the treatmentagent is preferably 45 to 65° C.

The surface of the aluminum-based metal component is generally broughtinto contact with the treatment agent by immersing the component in thetreatment agent. Alternatively, the treatment agent may be sprayed orapplied onto the surface of the aluminum-based metal component. Wherethe treatment agent is maintained at the aforementioned preferredtemperature (45 to 65° C.), the aluminum-based metal component ispreferably kept in contact with the treatment agent for a period(contact period) of 1 to 10 minutes by the immersion or the like.

After the film is chemically formed on the surface of the aluminum-basedmetal component by the immersion or the like, the surface of thecomponent is rinsed with water, rinsed with hot water and dried ifnecessary.

The aluminum-based metal component generally has an oxide film on itssurface. In the case of a conventional chemical treatment using aphosphate of an alkali metal, it is necessary to remove the oxide filmby etching prior to the chemical treatment. In the present invention,ammonium fluoride, which is useful for etching away an aluminum oxidefilm, is contained in the treatment agent, so that the oxide film on thesurface of the aluminum-based metal component is dissolved by thetreatment agent. This eliminates the need for the etching step, which isotherwise required in the conventional method. However, the etching stepmay be performed if necessary.

The film thus chemically formed is colored yellow or sunny yellow asdescribed above. Therefore, whether or not the film is present on thecomponent can be checked at a glance. This makes it easy to check thepresence or absence of the film, thereby improving the efficiency of theproduction control in the production process for a corrosion-proofaluminum-based metal product.

Aluminum and the aluminum alloy as the material for the aluminum-basedmetal component are not particularly limited, but examples thereofinclude all aluminum-based metals including No. 1000 to No. 7000 series,AC series and ADC series. The corrosion-proof aluminum-based metalproduct is not particularly limited, but examples thereof includeautomotive components such as engine mounts, electrical components suchas camera bodies, constructive components such as aluminum windowsashes, office furniture such as desks, and other products which areused in various fields.

Next, examples of the present invention will be described in conjunctionwith a prior art example. It should be understood that the invention benot limited to these examples.

EXAMPLE 1 Aluminum-Based Metal Component

A plate (25 mm×60 mm×3 mm (thickness)) of an aluminum alloy A6063 wasprepared.

Treatment Agent

A treatment agent for chemical film formation was prepared by mixing35.0 g of a zinc sulfate aqueous solution (containing 15wt % of zincsulfate heptahydrate), 15.0 g of an ammonium fluoride aqueous solution(containing 10 wt % of ammonium fluoride) and 10.0 g of a sodiummolybdate aqueous solution (containing 5 wt % of sodium molybdatedehydrate), and diluting the resulting mixture so that zinc sulfateheptahydrate, ammonium fluoride and sodium molybdate dihydrate werepresent in concentrations of 5.25 g/liter, 1.5 g/liter and 0.5 g/liter,respectively, in the treatment agent.

Chemical Film Formation

The aluminum-based metal component was immersed in the treatment agentat 75° C. for 20 minutes, whereby a film was chemically formed on asurface of the aluminum-based metal component. Thereafter, the resultingcomponent was taken out of the treatment agent, rinsed with water,rinsed with hot water and dried. Thus, a corrosion-proof aluminum-basedmetal product was produced.

EXAMPLE 2 Treatment Agent

A treatment agent for chemical film formation was prepared by mixing52.5 g of a zinc sulfate aqueous solution (containing 15 wt % of zincsulfate heptahydrate), 22.5 g of a ammonium fluoride aqueous solution(containing 10 wt % of ammonium fluoride) and 15.0 g of a sodiummolybdate aqueous solution (containing 5 wt % of sodium molybdatedihydrate), and diluting the resulting mixture so that zinc sulfateheptahydrate, ammonium fluoride and sodium molybdate dihydrate werepresent in concentrations of 7.875 g/liter, 2.25 g/liter and 0.75g/liter, respectively, in the treatment agent.

Chemical Film Formation

The aluminum-based metal component was immersed in the treatment agentat 65° C. for 10 minutes, whereby a film was chemically formed on asurface of the aluminum-based metal component. Thereafter, the componentwas taken out of the treatment agent, rinsed with water, rinsed with hotwater and dried. Thus, a corrosion-proof aluminum-based metal productwas produced.

EXAMPLE 3 Treatment Agent

A treatment agent for chemical film formation was prepared by mixing70.0 g of a zinc sulfate aqueous solution (containing 15 wt % of zincsulfate heptahydrate), 30.0 g of a ammonium fluoride aqueous solution(containing 10 wt % of ammonium fluoride) and 20.0 g of a sodiummolybdate aqueous solution (containing 5 wt % of sodium molybdatedihydrate), and diluting the resulting mixture so that zinc sulfateheptahydrate, ammonium fluoride and sodium molybdate dihydrate werepresent in concentrations of 10.5 g/liter, 3.0 g/liter and 1.0 g/liter,respectively, in the treatment agent.

Chemical Film Formation

The aluminum-based metal component was immersed in the treatment agentat 55° C. for 5 minutes, whereby a film was chemically formed on asurface of the aluminum-based metal component. Thereafter, the componentwas taken out of the treatment agent, rinsed with water, rinsed with hotwater and dried. Thus, a corrosion-proof aluminum-based metal productwas produced.

EXAMPLE 4 Treatment Agent

A treatment agent for chemical film formation was prepared by mixing87.5 g of a zinc sulfate aqueous solution (containing 15 wt % of zincsulfate heptahydrate), 37.5 g of a ammonium fluoride aqueous solution(containing 10 wt % of ammonium fluoride) and 25.0 g of a sodiummolybdate aqueous solution (containing 5 wt % of sodium molybdatedihydrate), and diluting the resulting mixture so that zinc sulfateheptahydrate, ammonium fluoride and sodium molybdate dihydrate werepresent in concentrations of 13.125 g/liter, 3.75 g/liter and 1.25g/liter, respectively, in the treatment agent.

Chemical Film Formation

The aluminum-based metal component was immersed in the treatment agentat 45° C. for 1 minute, whereby a film was chemically formed on asurface of the aluminum-based metal component. Thereafter, the componentwas taken out of the treatment agent, rinsed with water, rinsed with hotwater and dried. Thus, a corrosion-proof aluminum-based metal productwas produced.

EXAMPLE 5 Treatment Agent

A treatment agent for chemical film formation was prepared by mixing105.0 g of a zinc sulfate aqueous solution (containing 15 wt % of zincsulfate heptahydrate), 45.0 g of a ammonium fluoride aqueous solution(containing 10 wt % of ammonium fluoride) and 30.0 g of a sodiummolybdate aqueous solution (containing 5 wt % of sodium molybdatedihydrate), and diluting the resulting mixture so that zinc sulfateheptahydrate, ammonium fluoride and sodium molybdate dihydrate werepresent in concentrations of 15.75 g/liter, 4.5 g/liter and 1.5 g/liter,respectively, in the treatment agent.

Chemical Film Formation

The aluminum-based metal component was immersed in the treatment agentat 35° C. for 30 seconds (0.5 minute), whereby a film was chemicallyformed on a surface of the aluminum-based metal component. Thereafter,the component was taken out of the treatment agent, rinsed with water,rinsed with hot water and dried. Thus, a corrosion-proof aluminum-basedmetal product was produced.

EXAMPLE 6 Treatment Agent

A treatment agent for chemical film formation was prepared by mixing175.0 g of a zinc sulfate aqueous solution (containing 15 wt % of zincsulfate heptahydrate), 45.0 g of a ammonium fluoride aqueous solution(containing 10 wt % of ammonium fluoride) and 50.0 g of a sodiummolybdate aqueous solution (containing 5 wt % of sodium molybdatedihydrate), and diluting the resulting mixture so that zinc sulfateheptahydrate, ammonium fluoride and sodium molybdate dihydrate werepresent in concentrations of 26.25 g/liter, 4.5 g/liter and 2.5 g/liter,respectively, in the treatment agent.

Chemical Film Formation

The aluminum-based metal component was immersed in the treatment agentat 35° C. for 30 seconds (0.5 minute), whereby a film was chemicallyformed on a surface of the aluminum-based metal component. Thereafter,the component was taken out of the treatment agent, rinsed with water,rinsed with hot water and dried. Thus, a corrosion-proof aluminum-basedmetal product was produced.

PRIOR ART EXAMPLE 1

The surface of the aluminum-based metal component was subjected to anetching process, and then to a chemical film formation process employinga phosphate of an alkali metal in the following manner.

Etching Process

An acidic etching agent JCB-7030 (available from Japan C. S. ChemicalCo., Ltd.). was diluted into a concentration of 200 g/liter with water,and the aluminum-based metal component was immersed in the resultingetching agent at 60° C. for 5 minutes.

Chemical Film Formation Process

CHEMIBONDER 5701 (Japan C. B. Chemical Co., Ltd.) essentially containingsodium dihydrogen phosphate was diluted into a concentration of 120g/liter with water, and the etched aluminum-based metal component wasimmersed in the resulting treatment agent at 60° C. for 5 minutes,whereby a film was chemically formed on the surface of the component.Then, the component was taken out of the treatment agent, rinsed withwater, rinsed with hot water and dried. Thus, a corrosion-proofaluminum-based metal product was produced.

Check of Chemically Formed Films

The corrosion-proof aluminum-based metal products produced in Examples 1to 6 and Prior Art Example 1 were each visually checked for the presenceor absence of a chemically formed film. As a result, it was found thatthe surfaces of the products of Examples 1 to 6 were colored yellow orsunny yellow. Therefore, the formation of the films was detected at aglance. On the other hand, the chemically formed film of thecorrosion-proof aluminum-based metal product of Prior Art Example 1 wasnot visually detected, because the surface of the product was colorlessand transparent. The surface of the corrosion-proof aluminum-based metalproduct of Prior Art Example 1 was analyzed by an ICP emissionspectrometer (OPTIMA 4300DV available from Perkin Elmer Corporation)and, as a result, it was confirmed that the product had a chemicallyformed film in a surface thereof. The corrosion-proof aluminum-basedmetal products of Examples 1 to 6 were also analyzed by the ICP emissionspectrometer. The analysis indicates that the products each had achemically formed yellow or sunny yellow film in a surface thereof.Further, the compositions (content ratio) of the films were analyzed.The results of the analysis are shown in Table 1.

TABLE 1 Example Prior Art 1 2 3 4 5 6 Example 1 Treatment agent ZnSO₄aq. solution (g) 35.0 52.5 70.0 87.5 105.0 175.0 NaH₂PO₄ ZnSO₄•7H₂O(g/L) 5.25 7.875 10.5 13.125 15.75 26.25 120.0 g/L NH₄F aq. solution15.0 22.5 30.0 37.5 45.0 45.0 (g) NH₄F (g/L) 1.5 2.25 3.0 3.75 4.5 4.5Na₂MoO₄ aq. solution (g) 10.0 15.0 20.0 25.0 30.0 50.0 Na₂MoO₄•2H₂O(g/L) 0.5 0.75 1.0 1.25 1.5 2.5 Temperature (° C.) 75 65 55 46 35 35 60Immersion period (min) 20 10 5 1 0.5 0.5  5 Chemically formed film ColorYellow Sunny yellow Colorless Zn/Mo/Al 5/4/1 —

1. A method of preventing corrosion of an aluminum-based metal componentmade of aluminum or an aluminum alloy, the method comprising: bringing atreatment agent (A) into contact with a surface of the aluminum-basedmetal component to chemically form a film (B) on the surface of thealuminum-based metal component; the treatment agent (A) comprising anaqueous solution of zinc sulfate, an aqueous solution of ammoniumfluoride and an aqueous solution of sodium molybdate which are mixedtogether; the chemically formed film (B) comprising zinc fluoride,molybdenum and aluminum fluoride.
 2. A method as set forth in claim 1,wherein the treatment agent (A) contains 5.25 to 26.25 g/liter of zincsulfate heptahydrate, 1.5 to 4.5 g/liter of ammonium fluoride, and 0.5to 2.5 g/liter of sodium molybdate dihydrate.
 3. A method as set forthin claim 1, wherein the treatment agent (A) comprises the following zincsulfate aqueous solution (a), the following ammonium fluoride aqueoussolution (b) and the following sodium molybdate aqueous solution (c)which are mixed in a mass ratio of (a)/(b)/(c)=7/3/2: (a) an aqueoussolution containing 15% by weight of zinc sulfate heptahydrate; (b) anaqueous solution containing 10% by weight of ammonium fluoride; and (c)an aqueous solution containing 5% by weight of sodium molybdatedihydrate, wherein zinc of zinc fluoride, molybdenum and aluminum ofaluminum fluoride are present in the chemically formed film (B) in acontent ratio of zinc/molybdenum/aluminum=5/4/1 on a mass basis.
 4. Amethod as set forth in claim 3, wherein the treatment agent (A) having amass ratio of (a)/(b)/(c)=7/3/2 comprises 35 to 175 g of the solution(a), 15 to 45 g of the solution (b) and 10 to 50 g of the solution (c).5. A method as set forth in claim 1, wherein the treatment agent (A) hasa temperature of 45 to 65° C. and the surface of the aluminum-basedmetal component is kept in contact with the treatment agent (A) for aperiod of 1 to 10 minutes.
 6. A corrosion-proof aluminum-based metalproduct comprising: an aluminum-based metal component made of aluminumor an aluminum alloy; and a film (B) chemically formed on a surface ofthe aluminum-based metal component by a method as recited in claim 1,the film (B) comprising zinc fluoride, molybdenum and aluminum fluoride.7. A corrosion-proof aluminum-based metal product as set forth in claim6, wherein zinc of zinc fluoride, molybdenum and aluminum of aluminumfluoride are present in the chemically formed film (B) in a contentratio of zinc/molybdenum/aluminum=5/4/1 on a mass basis.